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air conditioning,

mechanical process for controlling the humidity, temperature, cleanliness, and circulation of air in buildings and rooms. Indoor air is conditioned and regulated to maintain the temperature-humidity ratio that is most comfortable and healthful. In the process, dust, soot, and pollen are filtered out, and the air may be sterilized, as is sometimes done in hospitals and public places.

Most air-conditioning units operate by ducting air across the colder, heat-absorbing side of a refrigeration apparatus and directing it back into the air-conditioned space (see refrigerationrefrigeration,
process for drawing heat from substances to lower their temperature, often for purposes of preservation. Refrigeration in its modern, portable form also depends on insulating materials that are thin yet effective.
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). The refrigeration apparatus is controlled by some form of thermostat. In water-cooled air-conditioning units, the waste heat is carried away by a flow of water. For recirculation in water-cooled units, a cooling tower is used. This apparatus maintains a constant level of water in the system and replaces water lost by evaporation. The development of small self-contained systems has greatly expanded the use of air conditioning in homes. A portable or window-mounted air conditioner is usually adequate for one room.

Often domestic heating systems are converted to provide complete air conditioning for a home. Usually, this is done by combining a heating device and a cooling device in one unit. In regions where the outside temperature does not fall too low, heat pumps have become popular. A heat pump is a reversible device that does mechanical work to extract heat from a cooler place and deliver heat to a warmer place. The heat delivered to the warmer place is, approximately, the sum of the original heat and the work done. Greater temperature differences between the warm and cold regions require greater amounts of work. In warm weather the heat pump acts like a traditional air conditioner, removing heat from the indoors and delivering heat to the outdoors. In cool weather, it removes heat from the outdoors and delivers heat to the indoors. The efficiency of a heat pump as a heating device depends upon the outdoor temperature. At 50°F; (10°C;) a heat pump is more efficient than a traditional heating system. Below 32°F; (0°C;) it is less efficient and requires augmenting with conventional heaters.

In the construction of office buildings in the United States, air-conditioning systems are commonly included as integral parts of the structure. First used c.1900 in the textile industry, air conditioning found little use outside factories until the late 1920s. It is of great importance in chemical, pharmaceutical, and other industrial plants where air contamination, humidity, and temperature affect manufacturing processes.


See D. Abrams, Low Energy Cooling (1988); S. Aglow, Electronic HVAC Controls Simplified (1988).

Air conditioning

A system that extracts heat from an area using a refrigeration cycle and treats the air to meet the requirements of a conditioned space by controlling its temperature, humidity, cleanliness, and distribution. A complete system of heating, ventilation, and air conditioning is referred to as HVAC.

Air Conditioning


The term “air conditioning” usually means the automatic establishment and maintenance in closed rooms or vehicles of certain properties of the air—temperature, relative humidity, purity, composition, velocity of motion, and pressure—which are most favorable for the well-being of humans (comfort air conditioning) or for production processes, operation of equipment and instruments, or preservation of valuable cultural objects and works of art (industrial air conditioning). Air conditioning systems frequently function as forced ventilation systems. During warm seasons they cool and dehumidify the air; during the cold season the air is heated and humidified. Air-conditioning systems may operate in combination with heating systems or perform their functions. Industrial air conditioning is usually used in insulated closed rooms, capsules, or containers.

Air conditioning was first used in the late 19th century in the textile industry. A patent for one of the main devices for humidification, dehumidification, and cooling of air by water—the air washer—was granted in the USA in 1897. The current method of regulating air humidity according to the dew point (moisture content) of the air leaving the air washer was proposed in 1906. Comfort air conditioning is thought to have first been used in 1922 in motion-picture theaters. The manufacture of “self-contained” air conditioners began in the 1930’s.

In the USSR air conditioning was first used in the 1920’s, mainly in the textile industry and later in other areas. Factory production of air conditioners and equipment for air-conditioning was organized during the early 1950’s.

Air conditioning for comfort is used mainly in public and office buildings and trains, aircraft, motor vehicles, and ships; it is used in industrial buildings in manufacturing a product that must meet prescribed standards, as well as to provide optimum working conditions, to increase labor efficiency, and to reduce spoilage. It is used in residential buildings, principally in southern regions. In mines and pits and in the cabs of cranes, tractors, combines, and excavators, it is used to improve working conditions; in scientific research institutions, for conducting experimental work under strictly defined, reproducible climatic conditions; and in libraries, archives, museums, and art galleries, to provide the most favorable conditions for preserving books, documents, and works of art. In storehouses for food and agricultural products, air conditioning creates the microclimate necessary to minimize losses and preserve the taste and nutritive properties of stored products. In medical institutions air conditioning is used to maintain the required air purity and for therapeutic purposes. In agricultural buildings it is used for year-round growing of vegetables, fruits, and flowers and for raising and fattening fowl.

Industrial air conditioning is used to maintain a constant moisture content of materials and rate of chemical and biochemical reactions and crystallization processes; it also maintains the temperature and relative humidity required for testing materials under standardized conditions. It is required in rooms where hygroscopic materials are manufactured and treated, since air temperature and relative humidity greatly influence technological processes and the weight, appearance, and quality of the materials and of articles manufactured from them. A constant temperature is mandatory in the manufacture of precision tools and instruments with tolerances of the order of 2–3 microns, since fluctuations in temperature cause unacceptable variations in the size of the parts being manufactured. The relative humidity of the air (above 55 percent) that is maintained in certain manufacturing buildings virtually eliminates the possibility of accumulation of electrostatic charges, which is of particular importance where explosive mediums are used. Air conditioning is used in more than 200 production processes in various branches of industry. In a number of cases the temperature level inside an industrial building is determined by comfort conditions.

The optimum comfort properties of air that satisfy sanitary and health requirements are listed in Table 1; in the USSR the parameters are regulated by the Construction Standards and Rules.

An air-conditioning system is the aggregate of apparatus for the treatment, movement, and distribution of the air and the automatic regulation of its properties.

Air-conditioning systems are equipped with apparatus for dust removal and heating, cooling, humidification, and dehumidification of air, as well as for automatic control of the properties of the air. In some cases air-conditioning systems also impart an odor to the air or deodorize it, regulate its ionic composition (ionization), remove excess carbon dioxide, enrich it with oxygen (regeneration), and clean it bacteriologically.

A distinction is made between central air-conditioning systems and individual units. Central systems usually cool several rooms; individual units cool a single room or part of a room. Central systems have outside sources of heat (provided by hot water, steam, gas, or electricity), cooling (provided by cold water from refrigeration equipment, artesian wells, and mountain streams or by a refrigerant or brine), and electric power to drive the motors for pumps, fans, and compressors. Individual units can have their own built-in cooling sources and be supplied externally only with electric power required for refrigeration machines, pumps, and fans and for electric air heaters and humidifiers. In areas with a dry hot climate, air cooling by water evaporation (evaporative cooling) can be used.

According to the air pressure provided by the forcing fans, air-conditioning systems may be classified as low-pressure (up to 1 kilonewton per sq m [kN/m2], or 100 kilograms-force per sq m [kgf/m2]), medium-pressure (1–3 kN/m2), and high-pressure (more than 3 kN/m2). Central and individual air-conditioning systems may be of the single-duct or two-duct type. A single-duct low-pressure system includes air-treatment equipment (an air conditioner), ducts and equipment for supplying conditioned air and removing exhaust air, and apparatus for automatic regulation and remote control. Central air-water systems (usually the single-duct type) are supplied with heating and cooling from stations through pipeline systems consisting of one, two, three, or four pipes. Air-conditioning systems can be of the flow-through type or with partial recirculation of room air. In a flow-through system only the outside air is treated and circulated. In systems with partial recirculation, the air being treated and circulated is a mixture of outside air and room air drawn from the area being cooled (to economize on heat during the cold season and cooling during the warm season). The air temperature and humidity required in a room are provided by the appropriate heaters and dehumidifiers or humidifiers, respectively. During the warm season, air-conditioning systems sometimes operate in conjunction with radiant cooling systems. The removal and recirculation of exhaust air and, in a number of cases, the supply of conditioned air through fluorescent lighting fixtures began in the 1960’s. The heat released by the lighting fixtures is thus used for heating the air.

Single-duct systems are made with centralized regulation of air quality, with centralized or local regulation of quantity, and with local or zonal terminal units. Single-duct medium-pressure systems with induction-type terminal units mounted under the windows are commonly used in high-rise office buildings and hotels. The operation of such systems yields some savings in heating and cooling, since the inside air can be recirculated within a room without a central recirculation system, which requires the installation of recirculation ducts. Two-duct systems now being developed have one duct for cold air and another for warm. In or near each room there is a special mixing chamber in which cold and warm air are combined in the required proportion. The mixing is controlled by a thermostatic regulator, and air enters the room at the required temperature.

The air-conditioning systems in railroad cars, buses, automobiles, and aircraft are usually of the single-duct type and the cooling is supplied by built-in refrigeration machines. Ship air-conditioning systems may be of the single-duct or two-duct type, operating at medium or high pressure.

The installation of air conditioning in a building usually leads to increased construction or alteration costs. However, in industrial buildings such expenses are often justified by increased labor productivity, improved quality of the product, and reduced spoilage. Today the operation of many industrial plants and shops would be virtually impossible without air conditioning— for example, in the textile, tobacco, pharmaceutical, and electrovacuum industries and in manufacturing precision instruments and machines.

In the future, air conditioning will be widely used. Factors contributing to this trend include the development of new manufacturing processes that are strongly dependent on stable conditions of the air; the increasing use in industrial plants, scientific research institutions, and design organizations of expensive precision instruments, mechanisms, and computers whose trouble-free operation is possible only at a certain temperature and humidity; the increase in construction of enclosed public areas intended for long-term occupancy by large numbers of persons (since year-round availability is required for profitable operation of such areas); the use of large areas of glass in construction; and the absence of windows and skylights in some types of industrial buildings.


Zvorykin, M. L., and V. M. Cherkez. Ustanovki konditsionirovaniia vozdukha i kholodil’niki passazhirskikh vagonov. Moscow, 1969.
Barkalov, B. V., and E. E. Karpis. Konditsionirovanie vozdukha v promyshlennykh, obshchestvennykh i zhilykh zdaniiakh. Moscow, 1971.
Table 1. Standardized optimal parameters for air in the working areas of industrial buildings (at constant work positions) and in air-conditioned zones of public and residential buildings
 Cold and transitional seasons (outside air temperature below 10°C)Warm seasons (outside air temperature 10°C and higher)
 Type of workTemperature (°C)Relative humidity (percent)Maximum speed (m/sec)Temperature (°C)Relative humidity (percent)Maximum speed (m/sec)
Industrial buildings. . . . . . . . . .Light18-210.222-25
Residential and public buildings. . . . . . . . . .19-2160-400.322-2560-400.3
Zakharov, Iu. V., and L. M. Andreev. Oborudovanie sudovykh sistem konditsionirovaniia vozdukha. Leningrad, 1971.


air conditioning

[′er ‚kən′dish·ən·iŋ]
(mechanical engineering)
The maintenance of certain aspects of the environment within a defined space to facilitate the function of that space; aspects controlled include air temperature and motion, radiant heat level, moisture, and concentration of pollutants such as dust, microorganisms, and gases. Also known as climate control.
A chemical process by which small fibers are sealed into yarn.

Air conditioning

The control of certain environmental conditions including air temperature, air motion, moisture level, radiant heat energy level, dust, various pollutants, and microorganisms.

Comfort air conditioning refers to control of spaces to promote the comfort, health, or productivity of the inhabitants. Spaces in which air is conditioned for comfort include residences, offices, institutions, sports arenas, hotels, factory work areas, and motor vehicles. Process air-conditioning systems are designed to facilitate the functioning of a production, manufacturing, or operational activity.

A comfort air-conditioning system is designed to help maintain body temperature at its normal level without undue stress and to provide an atmosphere which is healthy to breathe. The heat-dissipating factors of temperature, humidity, air motion, and radiant heat flow must be considered simultaneously. Within limits, the same amount of comfort (or, more objectively, of heat-dissipating ability) is the result of a combination of these factors in an enclosure. Conditions for constant comfort are related to the operative temperature. The perception of comfort is related to one's metabolic heat production, the transfer of this heat to the environment, and the resulting physiological adjustments and body temperature.

Engineering of an air-conditioning system starts with selection of design conditions; air temperature and relative humidity are principal factors. Next, loads on the system are calculated. Finally, equipment is selected and sized to perform the indicated functions and to carry the estimated loads.

Each space is analyzed separately. A cooling load will exist when the sum of heat released within the space and transmitted to the space is greater than the loss of heat from the space. A heating load occurs when the heat generated within the space is less than the loss of heat from it. Similar considerations apply to moisture.

The rate at which heat is conducted through the building envelope is a function of the temperature difference across the envelope and the thermal resistance of the envelope (R value). Overall R values depend on materials of construction and their thickness along the path of heat flow, and air spaces with or without reflectances and emittances, and are evaluated for walls and roofs exposed to outdoors, and basements or slab exposed to earth. In some cases, thermal insulations may be added to increase the R value of the envelope.

Solar heat loads are an especially important part of load calculation because they represent a large percentage of heat gain through walls, windows, and roofs, but are very difficult to estimate because solar irradiation is constantly changing.

Humidity as a load on an air-conditioning system is treated by the engineer in terms of its latent heat, that is, the heat required to condense or evaporate the moisture, approximately 1000 Btu/lb (2324 kilojoules/kg) of moisture. People at rest or at light work generate about 200 Btu/h (586 W). Steaming from kitchen activities and moisture generated as a product of combustion of gas flames, or from all drying processes, must be calculated. As with heat, moisture travels through the space envelope, and its rate of transfer is calculated as a function of the difference in vapor pressure across the space envelope and the permeance of the envelope construction. See Humidity control

A complete air-conditioning system is capable of adding and removing heat and moisture and of filtering airborne substitutes, such as dust and odorants, from the space or spaces it serves. Systems that heat, humidify, and filter only, for control of comfort in winter, are called winter air-conditioning systems; those that cool, dehumidify, and filter only are called summer air-conditioning systems, provided they are fitted with proper controls to maintain design levels of temperature, relative humidity, and air purity. See Air filter

Built-up or field-erected systems are composed of factory-built subassemblies interconnected by means such as piping, wiring, and ducting during final assembly on the building site. Their capacities range up to thousands of tons of refrigeration and millions of Btu per hour of heating. Most large buildings are so conditioned.

There are three principal types of central air-conditioning systems: all-air, all-water, and air-processed in a central air-handling apparatus. In one type of all-air system, called dual-duct, warm air and chilled air are supplied to a blending or mixing unit in each space. In a single-duct all-air system, air is supplied at a temperature for the space requiring the coldest air, then reheated by steam or electric or hot-water coils in each space.

air conditioning

1. The process of treating air so as to control simultaneously its temperature, humidity, cleanliness, and distribution within an interior space such as a room or building.
2. Same as definition 1, but also controlling odor and noise.

air conditioning

a system or process for controlling the temperature and sometimes the humidity and purity of the air in a house, etc.
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